1. Field of the Invention
The present invention relates to a link device or a linkage for stretching a sheet material, for instance a thermo-plastic synthetic resin film, in the longitudinal, transverse or biaxial direction and to a stretching apparatus using said link device.
2. Description of the Prior Art
Biaxial simultaneous stretching apparatuses using a link mechanism for a continuous synthetic resin film are proposed in Japanese Patent Publication No. SHO 43-5560 and Japanese Patent Publication No. SHO 44-7155.
The stretching apparatus disclosed in Japanese Patent Publication No. SHO 43-5560 has a pair of stretching devices provided at opposite sides along the longitudinal direction of the stretching apparatus. Each of the stretching devices comprises an endless link device comprising a plurality of link members arranged to form a pantograph structure, a plurality of sliding shoes provided at the bottom side of the endless link device, means for circulating the endless link device and a rail unit to slidably guide the sliding shoes. The means for circulating the endless link device comprises a first sprocket provided at the entrance of the stretching apparatus and a second sprocket provided at the exit of the stretching apparatus. The endless link device is meshed with the sprockets. The rail unit is provided between a position where the endless link device leaves from the first sprocket and a position where the endless link device connects with the second sprocket. The rail unit has an outside rail, a central rail and an inside rail. A first gauge between the outside and inside rails becomes narrow in the direction from the first sprocket to the second sprocket to reduce the height of and to expand the length of the pantograph structure for applying a longitudinal stretching action to the film. The sliding shoes comprise a series of outside shoes slidably in contact with the outside rail, a series of central shoes slidably in contact with the central rail and a series of inside shoes slidably in contact with the inside rail. Each of the shoes is in contact with each of the rails at the bottom and side surfaces thereof. The gauge between the rail units provided at both sides along the stretching apparatus becomes wide in the direction from the first sprocket to the second sprocket to apply a transverse stretching action to the film.
In the stretching apparatus disclosed in Japanese Patent Publication No. SHO 43-5560 mentioned above, an extremely large driving force is required on the sprocket, because the endless link device is supported movably on the rail via sliding shoes which slide in contact with the surface of the rail unit. In the stretching apparatus, moreover, the shoes are in surface contact with the rails at their side surfaces and are forcibly slid along the rails while moving from the entrance to the exit of the rails. This mechanism is a toggle mechanism and it brings about quite a large force acting between the rails and the shoes. Existence of such action runs counter to the expectation of stability of circulation of the endless link device at a high speed operation of the stretching apparatus. In the stretching apparatus, further, it is not possible to provide the rail unit around the sprockets because if the rail unit is provided around the sprockets, there occurs an interference between the force acting on each of the sprockets and the force acting on each of the shoes through the endless link device meshing with each of the sprockets. Thus, in the stretching apparatus, the rail unit is provided only between both of the sprockets. Such an arrangement of rail unit is not suitable for high speed operation of the stretching apparatus because the shoes have to engage on and have to disengage from the rail at the entrance of the sprocket and at the exit of the sprocket, respectively. In the stretching apparatus, furthermore, since a very large force acts on the rails as mentioned above, it is necessary to use rigidly built rails. When changing a stretching ratio in the longitudinal direction, it is necessary to change the gauge between the outside rail and the inside rail. Changing the gauge between the rigidly built rails on the rail unit cannot be accomplished with a simple mechanism for varying and adjusting the gauge. Accordingly, in the stretching apparatus, a rail unit is changed with another one having a different gauge between the outside rail and the inside rail for varying the stretching ratio. This word causes workers in a factory substantial problems. Actually, therefore, the factory has to prepare several stretching apparatuses having different gauges to produce stretched films having different streching ratios if the factory wants to do so.
Japanese Patent Publication No. SHO 44-7155 teaches a stretching apparatus similar to the stretching apparatus shown in Japanese Patent Publication No. SHO 43-5560 mentioned above except for rotatable rollers provided to support the endless link device on the rail unit instead of the sliding shoes. Each of the rotatable rollers for supporting the weight of the endless link device is constructed by a caster roller which is swingable in a horizontal plane. Also in this embodiment, the stability of circulation of the endless link device at high speed can not be accomplished, because the caster rollers receive centrifugal force during travel of the endless link device around the sprockets and the directions of the caster rollers become unstable at that time. This phenomenon was clearly observed on the practical operation of the stretching apparatus having the same construction disclosed in Japanese Patent Publication No. SHO 44-7155 at a speed of about 200 m/min. or more.